He said: "We take a background reading using the monitor against one of ourselves, find the background reading for the day, there's a factor then added onto that for a pass limit so that we know whether the animals are above or below the accepted level."

The FSA also ensures that contaminated food does not reach the table.

Sandy McDougall, the agency's head of contaminants, said there may be some animals in Scotland which approach being up to two-times the safety limit.

Chernobyl, where the worst nuclear disaster happened 20 years ago

But he said: "All of the sheep-meat leaving such restricted farms is monitored to ensure it is well below the safety limit."

The process involves gathering sheep once a year for testing and getting subsequent tests before animals can be sent for slaughter.

The National Farmers Union Scotland said it was time to revisit the science and review the situation.

But he said: "Maybe 20 years on it's time for the Scottish Executive and others to have a look and maybe do a little bit of work to see why are we left with this pocket of farms, where one or two drop off every year.

"Maybe it's time to take an assessment of what the risk is now."

The FSA said it reviewed the situation regularly, but there were too many factors involved to determine accurately when the restrictions would be lifted.

So far, more than £3m in compensation has been paid to farmers in Scotland.